We are investigating the natural history and in situ organization of the receptor for IgE on mast cells and basophils. Various sublines of rat basophilic leukemia (RBL) cells and plasma membrane vesicles prepared from them are used. Purified IgE myeloma, antibodies to IgE and to the receptor are also used. These studies will help clarify the mode of function of the receptor, the fate of the receptor-bound monomeric and cross-linked IgE and the turnover of the free receptor.